Loulabell
The most factor thing for a race like this is, to have a pair of shoes that are well tested and that you are comfortable with. It is best to use the pair that you have done most of your training mileage in.
I would use road shoes for an event like the Northants ultra, and the primary consideration in that, would be the fact that there are few places near to my home where you can practice long runs in trail shoes. By all means experiment with other shoes, but when the race comes and you have to choose your weapon, go with the most proven shoe in the cupboard.
The issue with junior trainers is based mainly on the assumption that a child's foot strikes the ground in a slightly different way. Children and adults of small stature are less likely to suffer mechanical problems in any event. The bigger they are the harder they fall!